Some image photographing devices may be classified as complementary metal oxide semiconductor (CMOS) image photographing devices or charge coupled image-photographing devices (CCD). CMOS image photographing devices convert optical signals, which are input when a subject is photographed, into digital signals and reproduce images of the subject on a screen using the digital signals.
FIG. 1 is a schematic block diagram of a conventional CMOS image-photographing device 101. Referring to FIG. 1, the CMOS image-photographing device 101 includes a pixel array 111 and an analog-to-digital converter (ADC) 121. The pixel array 111, which includes a plurality of pixels, outputs an optical signal from an external source as an analog signal AN1. The ADC 121 includes a ramp signal generator 131. The ADC 121 converts the analog signal AN1 into a digital signal D1 in response to a ramp signal Vramp (see FIG. 2) generated by the ramp signal generator 131 and outputs the digital signal D1.
FIG. 2 is a circuit diagram of the ramp signal generator 131 illustrated in FIG. 1. Referring to FIG. 2, the ramp signal generator 131 includes a resistor 211, a capacitor 221, and an operational amplifier 231. The operational amplifier 231 integrates an input signal Vin from an external source using a resistance of the resistor 211 and a capacitance of the capacitor 221 and outputs the integrated value as the ramp signal Vramp. Here, the ramp signal Vramp determines the gain characteristics of the ADC 121 of FIG. 1.
When fabrication/process margins are insufficient or there are dramatic manufacturing changes, the resistor 211 and the capacitor 221 may not be formed as designed. As a result, the gain characteristics of the CMOS image-photographing device 101 of FIG. 1 may be adversely influenced by process variations and this may result in lower device yield for the device 101.